The present invention relates to a system and method for processing a workpiece, such as a semiconductor workpiece, and more particularly relates to a system and method for controlling the temperature of the workpiece in a thermal processing system.
Thermal processing systems or furnaces have been widely known and used for many years to perform a variety of semiconductor fabrication processes, including annealing, diffusion, oxidation, and chemical vapor deposition. As a result, these processes are well understood, especially with regard to the impact of process variables on the quality and uniformity of resulting products. Thermal processing furnaces typically employ either a horizontal-type furnace or a vertical-type furnace. For some applications, vertical-type furnaces are preferred because they create less particles during use, thus decreasing the incidence of contamination and workpiece waste, they can be easily automated, and they require less floor space because of their relatively small footprint.
Both conventional types of furnaces are designed to heat semiconductor wafers to desired temperatures to promote either diffusion of implanted dopants to a desired depth or to perform other conventional processing techniques, such as the application of an oxide layer to the wafer or deposition of a chemical vapor layer to the wafer. The heating requirements of the wafer are generally important and thus are closely monitored.
Conventional vertical-type thermal processing furnaces, such as tube furnaces, are designed to support the processing tube within the furnace in the vertical position. The thermal furnace also typically employs a wafer boat assembly which is mounted to appropriate translation mechanisms for moving the wafer boat into and out of the processing tube. A wafer-handling assembly is deployed adjacent and parallel to the wafer-boat assembly to transfer the semiconductor wafers from wafer cassettes to the wafer-boat assembly. The wafers are then raised into a quartz or silicon heating tube. The tube is then slowly raised to the desired temperature and maintained at that temperature for some pre-determined period of time. Afterwards, the tube is slowly cooled, and the wafers removed from the tube to complete the processing. A drawback of this processing technique is that it places constraints on the time-at-temperature to which a wafer can be subjected. Conventional vertical furnaces of these and other types are shown and described in U.S. Pat. No. 5,217,501 of Fuse et al. and in U.S. Pat. No. 5,387,265 of Kakizaki et al.
Another problem with conventional thermal processing systems is that the heating or processing regimen to which the wafer is subjected does not closely match a standard pre-established heating profile due to system constraints. For example, prior systems employ multiple pyrometers to measure temperature at various furnace locations, and then use the multiple signals generated by the pyrometers to control operation of multiple heating units, such as lamps. The system cycles the lamps on and off according to the system""s heating requirements. This heating control arrangement introduces unwanted noise in the system and generally makes it difficult to process a wafer in accordance with the pre-established heating profile.
There thus exists a need in the art for a thermal processing system that accurately controls wafer heating in accordance with a temperature or heating profile.
Other general and more specific objects of the invention will in part be obvious and will in part appear from the drawings and description which follow.
The present invention achieves the foregoing with a system and method for processing a workpiece in a thermal processing furnace. According to the present invention, the method includes the steps of measuring the temperature of the workpiece in the thermal processing furnace, and based upon the intended temperature profile and the measured temperature of the workpiece, moving the workpiece through the furnace to heat process the workpiece generally according to the intended temperature profile.
According to one aspect, the invention includes the step of providing an intended temperature profile of the workpiece to be generally achieved during processing in the thermal processing furnace. The method also allows a user to determine one or more selected set points of the thermal processing furnace, which are then used to construct the intended temperature profile.
According to another aspect, the method of the invention employs the step of moving the workpiece through the furnace based upon the furnace set points, the intended temperature profile and the measured temperature of the workpiece. According to another aspect, the method includes the steps of providing a wafer processing recipe, generating from said recipe the intended temperature profile, and controlling movement of the workpiece in the furnace to heat the workpiece generally according to the temperature profile. The workpiece is moved by using an elevator subsystem in response to the measured temperature of the workpiece. The recipe can include one or more of a temperature ramp-up rate, a temperature ramp-down rate, and a soak temperature.
According to another aspect, the invention includes the steps of determining the emissivity of the workpiece, and based upon the emissivity of the workpiece, determining the temperature of the workpiece. The temperature can be measured by providing an emissivity compensated pyrometry system.
According to still another aspect, the workpiece temperature is determined with a feedback subsystem for detecting and measuring the wafer temperature. According to one practice, the temperature is measured by determining the emissivity of the wafer, generating an output signal with a pyrometer indicative of the wafer temperature, filtering the output signal, and based on the wafer emissivity and the output signal, determining the wafer temperature. A control facility is provided for controlling movement of the wafer within the thermal processing furnace in response to the measured wafer temperature. The control facility performs this function by determining the difference between the measured temperature and the temperature profile to form an error signal, and moving the wafer within the furnace with an elevator subsystem. The control facility can also optionally perform the steps of scaling the error value, sampling the temperature profile at one or more points, determining the boundary conditions of the elevator subsystem, and moving the wafer at a selected speed within the chamber.
The present invention also provides a system for processing a workpiece. The system includes a thermal processing furnace having a process chamber, a feedback subsystem coupled to the thermal processing furnace for detecting a parameter of the workpiece when disposed in the processing chamber, a control facility for generating a control signal based upon an intended temperature profile of the workpiece and the parameter of the workpiece, and an elevator subsystem in communication with the control facility for moving the workpiece through the process chamber in response to the control signal to heat process the workpiece generally according to the intended temperature profile. The control facility is adapted to control the elevator subsystem to move the workpiece along an intended path through the processing chamber as a function of the intended temperature profile.
The system can also include a user interface for providing a wafer processing recipe that includes a temperature ramp-up rate, a temperature ramp-down rate, and/or a soak temperature. The system can also employ a trajectory generator for generating the intended temperature profile in response to the recipe.
According to one aspect, the control facility generates the control signal to move the workpiece through the furnace based upon the furnace set points, the intended temperature profile, and the measured temperature of the workpiece. According to one practice, the measured temperature of the workpiece is determined using the wafer thermal properties.
According to another aspect, the feedback subsystem includes a pyrometer for measuring the temperature of the workpiece, an emissivity measurement stage for measuring the emissivity of the wafer, and a filtering stage for filtering one of the measured temperature and emissivity, wherein the feedback subsystem generates a signal corresponding to the filtered temperature of the workpiece.
According to another aspect, the system can also include a plurality of thermocouples for measuring the temperature of the furnace at one or more locations. A furnace modeling stage generates an output signal corresponding to the actual measured temperature of the thermocouples in response to an input signal generated by the thermocouples, which is representative of the thermal properties of the furnace. An optional wafer temperature modeling stage for generating one or more signals indicative of one or more parameters (such as the raw emissivity and temperature values) of the workpiece in response to a workpiece position signal generated by the elevator subsystem and the output signal of the furnace thermal modeling stage.
Other general and more specific objects of the invention will in part be obvious and will in part be evident from the drawings and description which follow.